This invention relates to an amplifier device for use with a condenser microphone.
FIGS. 4 and 5 show the circuit arrangements of typical conventional amplifiers for use with condenser microphones (hereinafter, referred to simply as microphones). These arrangements are different chiefly in their buffer amplifiers. These buffer amplifiers are of FET common-source type as in FIG. 4 and FET source-follower type as in FIG. 5. The general buffer amplifier is formed by combining a microphone capsule with such a buffer amplifier into a microphone unit. Each of the conventional arrangements will be described in order.
Referring to FIG. 4, there are shown a microphone capsule 1, and a common-source type buffer amplifier circuit 11 which is formed of a diode 2 and an FET 3. The output terminal of an amplifier 401 corresponds to an amplifier output terminal 15 to which the drain of the FET 3 is connected. This amplifier output terminal is also connected through a load resistor 43 to a ripple filter 402. The ripple filter 402 serves to remove noise such as ripples superimposed on a power source 5. In addition, there is shown a speaker amplifier 110 which is formed of a power amplifier circuit 6 and a loudspeaker 7. This speaker amplifier acts to amplify the output signal that is supplied from the buffer amplifier circuit 11 through the amplifier output terminal 15 and a ground terminal 14, and to convert the amplified signal into a sound through the loudspeaker 7.
The operation of this conventional arrangement will now be described. An acoustic pressure signal is converted into a voltage signal by the microphone capsule 1 and supplied to the gate of the FET 3 that is biased by the diode 2. This voltage signal is converted into a current signal by the mutual conductance of the FET 3. Since the drain of the FET 3 is connected through the load resistor 43 and ripple filter 402 to the power source, the current signal is again converted into a voltage signal by the load resistor 43. Since the drain current of the FET 3 is almost not changed with the change of the drain-source voltage because of its characteristics, it can be considered that approximately a current source is present at the drain output terminal. Eventually, the acoustic pressure signal applied to the microphone capsule 1 appears across the load resistor 43 in the form of a voltage signal proportional to its magnitude.
The audio voltage signal developed across the load resistor 43 is supplied to the input of the power amplifier circuit 6, thereby being amplified to an extent necessary for driving the loudspeaker. The power amplifier circuit 6 is constructed to operate on a single power source in order to simplify its arrangement. For this reason, one end of the input of the power amplifier circuit 6 to which the voltage signal is applied is connected to the minus side of the power source 5, or to the ground terminal 14. Therefore, it is necessary that the output signal produced from the buffer amplifier circuit 11 to appear across the load resistor 43 be generated between the amplifier output terminal 15 and the ground terminal 14.
The above condition can be satisfied by equivalently AC-grounding the power source to which the load resistor 43 is connected, or shunting it only for AC. For this purpose, the ripple filter 402 is provided to satisfactorily reduce the AC components superimposed on the power source 5. If the power source to the load resistor 43 includes a ripple, the sum of the audio signal developed across the load resistor 43 and the ripple voltage is supplied to the input terminal of the power amplifier circuit 6, incurring the increase of unnecessary noise.
The amplifier 401 including the common-source type buffer amplifier circuit 11 as shown in FIG. 4 must be thus constructed to fully reduce the AC noise superimposed on the power source since the acoustic pressure signal applied to the microphone capsule 1 appears across the load resistor 43 one end of which is connected to the power source.
In an amplifier 501 using an FET source-follower type buffer amplifier circuit 31 as shown in FIG. 5, the acoustic pressure signal applied to the microphone capsule 1 appears across a load resistor 33. Since one end of the load resistor is grounded, there is almost no effect of the AC noise of the power source and thus it is not necessary to provide the ripple filter 402. However, the operation to draw out the voltage signal in the source-follower type is different from that in the common-source type and thus the phases of the signals in both cases are opposite to each other. Although the acoustic pressure signal applied to the microphone capsule 1 has the same phase in both cases, the phase of the voltage signal produced from the output terminal 15 in one case is opposite to that in the other case. The buffer amplifier circuit 31 has a small output impedance because of the source-follower configuration and thus the drain output end can be approximately regarded as a voltage source.
This opposite-phase relation between both cases causes a problem when a plurality of microphones are connected to the speaker amplifier so that the signals therefrom are mixed. When the microphones of opposite phases, or of different types are used close to each other, the acoustic pressure signal from the same sound source is applied to those microphones, and thus the amplifiers of the microphones produce signals of opposite phases, respectively. Since those signals are mixed, particularly the low-frequency components of the signals cancel out each other, changing the tone of the produced signal. In order to avoid this problem, it is generally practiced to equalize the phases of the signals fed from the microphones to the amplifiers.
However, the conventional amplifier including the FET common-source type buffer amplifier circuit shown in FIG. 4, in which the output terminal of the buffer amplifier circuit is connected through the load resistor to the power source, is easily affected by the ripple of the power source and thus needs a ripple filter before the power source so that the AC components can be removed from the power source. This results in making the amplifier complicated in construction.
In addition, the amplifier using the common-source type buffer amplifier circuit shown in FIG. 4 and the amplifier using the FET source-follower type buffer amplifier circuit shown in FIG. 5 are required to produce signals of the same phase. Since the conventional small microphones cannot include such means for making the phases equal, phase-switching means must be separately provided in the apparatus to which these amplifiers will be probably connected. In this case, however, the microphone of which the phase should be inverted cannot be distinguished from the other microphones from the external appearance of that microphone. Thus, an additional operation for examining the types of the microphones to be used is necessary before achieving the proper phase adjustment among the microphones.